Cyclic Oxidation–Reduction Cleaning for Microstructural and Chemical Restoration of Copper after Chemical Mechanical Polishing

Achieving ultraclean copper (Cu) surfaces is essential for reliable Cu-to-Cu bonding in advanced packaging technologies. As feature dimensions continue to shrink, even minimal oxy-carbide (CuOxCy) residues can critically degrade the bonding yield and electrical performance. Conventional plasma or reduction-only cleaning processes typically leave ≥12 atom % oxy-carbide species, limiting surface purity. In this study, a cyclic oxidation–reduction cleaning strategy was developed and optimized using blanket chemical-mechanically polished (CMP) Cu films to compare three cyclic plasma-free contamination removal methods. The proposed mechanism suggests that surface carbides are first oxidized to oxides and subsequently reduced back to metallic Cu during the cyclic cleaning processes. All cleaning schemes employed two sequential oxidation and reduction steps, combining mild oxidants (air or molecular oxygen) with three reduction techniques, ultrahigh vacuum anneal (UHVA), formic acid vapor, or forming gas anneal (FGA). In-system X-ray photoelectron spectroscopy confirmed that the deep double oxidation (DDO) process, utilizing O2 as the oxidant and UHVA as the reducing step, achieved the lowest residual oxy-carbide species (∼2 at. %), compared with ∼10 at. % for the double formic acid vapor (DFAV) process and ∼12 at. % in previous reports. Atomic force microscopy results show that the RMS roughness of the CMP Cu surface increased from 0.7 to 1.8 nm after DDO treatment and from 0.5 to 1.0 nm after DFAV treatment, consistent with the recrystallization and grain growth of the Cu grains. Ex-situ electron backscatter diffraction (EBSD) revealed a 35–45% reduction in amorphous impurity coverage, confirming restoration of crystalline order following the removal of surface and subsurface CuOxCy. This work introduces a scalable, plasma-free route for subsurface Cu surface cleaning, enabling next-generation hybrid bonding integration.